Prince Leonard Casley, the former ruler of Hutt River Province, and the "border".

The Australian Tax Office has spent more than $80,000 in its latest legal fight against the self-proclaimed royal family of an invented principality in Western Australia's wheatfields.

Last month the founders of the Hutt River Province, self-proclaimed former sovereign Prince Leonard Casley and his son Arthur, lost legal action against the Australian Taxation Office over the payment of eight years' of income tax, worth more than $3 million.

ATO legal cost summaries, released under freedom of information laws, show two cases against the 91-year-old Prince Leonard, who abdicated from the throne he established for himself nearly 50 years ago, and his son, Arthur Wayne Casley, have cost taxpayers $81,865.91 to date.

The bill for representation by the Australian Government Solicitor and additional legal counsel topped $40,000 in the case, which focused on back taxes from June 2006 to 2013.

Prince Leonard Casley, the former ruler of Hutt River Province.

Hutt River, located about 520 kilometres north of Perth, is not recognised by the federal government or state of Western Australia.

It sought to secede from Australia in the early 1970s, amid a heated dispute between Prince Leonard and state government officials over limits on the amount of wheat that could be sold.

Prince Leonard announced he would abdicate in February this year, handing sovereignty over Hutt River to 59-year-old son Graeme Casley. Prince Leonard suffers from emphysema and said he wanted to ensure the future of the principality after his death.

In July 2013, his wife Princess Shirley Joy Casley died at 85.

Western Australia Supreme Court Justice Rene Le Miere dismissed moves for payment demands from the ATO to be set aside in June, ordering Prince Leonard to pay $2.7 million. Arthur Casley was ordered to pay $242,000.

The pair argued the court did not have jurisdiction to make rulings against the sovereign or citizens of the province.

The "border" of Hutt River Province.

Justice Le Miere slammed the Casley submissions to the court as "gobbledegook", and said their documents ranged from being "merely irrelevant" to "the bizarre".

Graeme Casley told the ABC the family would consider an appeal in WA or to the High Court in Canberra.

The Casleys maintain the principality, which has its own homemade currency and stamps, has never been part of Australia or the British Empire.

"When Dad had commercial ventures in Western Australia, he paid tax on those," he said.

"But tax on goods and services here with the principality, which is an independent country, we are surprised they would pursue that. Like most commercial ventures a lot of cash isn't just laying idle."

The legal action follows a similar case from 1977, in which repeated demands for tax payments from the ATO prompted a shortlived declaration of war against Australia, withdrawn after several days.

The royals have asked for public assistance and pro bono legal support. Their website lists a ministry of state, foreign affairs, a royal mint, cabinet office and "high representative to the United States of America".

Visitors and tourists from around the globe trek to Hutt River to have their passports stamped and purchase souvenirs including Hutt River dollars.